Imaging systems that image both mid-wave infrared (MWIR (3 μm to 5 μm)) and long-wave infrared (LWIR (8 μm to 12 μm)) enable improved imaging for a variety of applications, including military and certain commercial applications. An aperture filter that provides dual pass bands and dual F-numbers together with a dual-band focal plane array (FPA) photodetector can simultaneously have high detection efficiency in both the MWIR and LWIR and thus provide the basis for imaging systems that generate images which include image data from both of these bands. Current high-performance MWIR/LWIR sensors require cryogenic cooling of the detector and limiting aperture to limit dark current and its associated noise, and a spectral filter to limit the background radiation reaching the FPA. Included are cold shield apertures within a cryogenically cooled dewar to limit received background radiation. The cold shield aperture also defines an effective F-number for the imaging system.
The F-number of the cold shield aperture is defined as the ratio of the distance (d) from the cold shield aperture to the focal plane of the FPA to the diameter (D) of the cold shield aperture. Due to wavelength differences between the bands, different F-numbers are needed to provide the same beam spot size on the FPA to achieve high image resolution for both the MWIR and LWIR bands. Thus, an important attribute for advanced MWIR/LWIR cameras is the incorporation of an in-dewar mechanism to permit the cold shield aperture to be varied in size (e.g. diameter) between two (or more) pre-determined size settings while maintaining near-100% shielding efficiency, as the system's F-number is varied to optimize performance in each band.
Conventional approaches for varying the F-number generally employ mechanical iris configurations that are either external or internal to the dewar that provide a first F-number during certain time instants for imaging one band, and a different F-number during other time instants to image the other band. Such mechanical approaches generally require complicated control electronics to implement the mechanical switching, are costly in terms of initial development and added per unit cost, decrease reliability, have inherent repeatability issues, and add thermal mass, which adds heat load to the imaging system that results in a requirement for higher capacity coolers. Moreover, such approaches do not provide temporally simultaneous imaging of the respective bands.